It’s not a pretty picture. Rather than “a very healthy reef full of diverse coral communities” including hundreds of fish, said marine biologist Ian Enochs, there’s just a thick carpet of green fuzz along with a few stunted corals: “The submarine landscape is just coated with algae.”

Enochs is part of a scientific team that for the first time has been able to study the actual impact of high levels of carbon dioxide on coral reefs, thanks to an uninhabited volcanic island called Maug.

“Carbon dioxide is bubbling up through vents in the sea floor like champagne,” said Enochs, a scientist for a joint research program of the University of Miami and the National Oceanic and Atmospheric Administration. “The water is not super warm, there’s no lava, just gas vents releasing CO2 into the water.”

Healthy and diverse coral reefs found about a kilometer from the underwater volcanic vents on Maug Island. (Photo: Stephani Gordon/Open Boat Films/NOAA)

The supersaturation of CO2 has shifted the area’s marine chemistry sharply toward the low end of the pH scale. This process, called ocean acidification, makes it difficult for corals—as well as clams, oysters, and other shellfish—to build their skeletons.

“As you move away from the vents, the acidification signal goes down. At the same time, the coral goes up. It’s a spatial shift. You see these less desirable algae communities giving way to healthy coral communities,” Enochs said.

Maug offers a preview of what scientists say is happening as the burning of fossil fuels increases the amount of carbon dioxide absorbed by the world’s oceans. As the oceans become more acidic, tropical coral reefs, which provide revenue and food for 400 million people, will die.

“These vents create conditions similar to what’s calculated to happen by the end of the century, given current rates of fossil fuel consumption,” said Enochs.

Coral reefs are considered the most biologically diverse areas in the oceans, thanks to the feeding grounds, living quarters, and hiding places that their intricate folds of rock provide to fish, shrimp, crabs, shellfish, sharks, and other marine life.

“In these acidified environments you don’t see that,” said Enochs, who is the lead author of a new paper about Maug in the journal Nature Climate Change. “There are a couple corals eking by, but their growth is lower, and you don’t see that geological buildup of living rock that is so desirable for fish habitat and storm protection, and for divers to visit and see.”

Enochs said the study was part of a growing body of scientific work demonstrating “that anthropogenic CO2 has widespread ramifications for numerous ecosystems, particularly coral reefs.”

“I think that curbing fossil fuel production is a logical step if we want to avoid this bleak future,” he added, along with other coral conservation moves such as reining in overfishing, and curbing high-nitrogen runoff from farms and lawns into the sea.

“We need to understand that all of these things combined are what are killing off coral reefs and take a multilevel approach toward preserving and protecting them,” Enochs said.

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